Air cavity packages are used to minimize dielectric loading effects that limit the bandwidth of high-frequency semiconductor devices. In conventional surface-mount air cavity packages, the semiconductors are mounted on a substrate including metal geometries that conduct heat downward, from the bottom of the Integrated Circuit (IC) to the bottom of the package. High frequency electrical signals are typically distributed to the side of the package. The package is in turn mounted to an application board, which serves the dual purposes of (a) routing Radio Frequency (RF) signals and Direct Current (DC) power to the package and (b) conducting heat away from the package. Traditional laminate air cavity RF packages dissipate heat from the bottom side. The active semiconductor die, typically Microwave Monolithic Integrated Circuits (MMICs), are mounted on a substrate made of fiberglass material. The active side of the MMICs faces the air. The heat from the MMICs is removed downward: from the bottom side of the MMIC, through the die attach material, the top metal layer on the substrate, thermal vias in the substrate, and then to an external heat sink. An example conventional surface-mounted air cavity package is shown in FIG. 1.
FIG. 1 illustrates a conventional multi-channel air cavity laminate module with a lid and a bottom-side heat path. In the embodiment illustrated in FIG. 1, a conventional laminate air cavity package 10 is mounted to an application board 12, typically by soldering. The solder joints joining the conventional air cavity package 10 to the application board 12 are shown as black-filled rectangles in FIG. 1. Within the conventional laminate air cavity package 10, devices 14 are mounted to a substrate 16 that includes both electrical vias 18 and thermal vias 20. A single via may function both as an electrical via and a thermal via. The electrical vias 18 provide electrical connections between pads on the device 14 and traces on or within the application board 12. In one embodiment, the device 14 is connected to the electrical vias 18 via wire bonds 22. The thermal vias 20 provide a conductive heat path between the device 14 so that heat can be transferred away from the device 14 and into the application board 12, e.g., via a heat transfer path 24. This approach is referred to as “bottom-side” cooling. For bottom-side cooling, the application board 12 must absorb and dissipate heat as well as route the RF and DC signals. Because the devices 14 are mounted to the inner surface of the air cavity package 10 closest to the application board 12, this configuration is referred to as having “bottom-mounted” devices. A lid 26 covers the devices 14 and forms one or more air cavities.
Some application boards, however, are designed for RF signal and DC power routing only and cannot provide a suitable bottom-side heat path. For these application boards, the conventional bottom-side cooling approach shown in FIG. 1 is unworkable.
Another conventional approach directs heat flow upward away from the application board 12, an approach referred to as “top-side cooling.” Applications requiring top-side cooling have made use of specially constructed Ball Grid Array (BGA) packages in which the MMIC or other device is attached to a heat sink on the top of the package. An example of this is shown in FIG. 2.
FIG. 2 illustrates a conventional BGA package with top-side cooling. In FIG. 2, an air cavity laminate package 28 contains the device 14 mounted to a metal base 30 that functions to transfer heat away from the devices 14 to the surrounding air instead of going into the application board 12. A lid 32 covers the devices 14 and forms one or more air cavities. The lid 32 also provides the electrical vias 18 that electrically connect pads to the devices 14 to traces on or within the application board 12. In the embodiment illustrated in FIG. 2, the laminate package 28 is attached to the application board 12 via solder balls, which are shown as black-filled circles in FIG. 2. The electrical vias 18 may be connected to the pads of the devices 14 via the wire bonds 22. Because the devices 14 are mounted to the inner surface of the package that is farthest from the application board 12, this configuration is referred to as having “top-mounted” devices.
Due to the position of the devices 14 at some distance away from the application board 12, however, the electrical vias 18 must be routed horizontally from the pins of the device 14 to the vertical portions of the lid 32, down through those vertical portions of the lid 32, and horizontally again to the application board 12. This creates electrical vias 18 that are relatively long, which results in increased inductance (L), resistance (R), and/or capacitance (C). RF circuits particularly may not be able to tolerate the additional L, R, or C, in which case the BGA package shown in FIG. 2 is unsuitable for higher frequency RF modules.
Thus, there is a need for air cavity laminate packages with top-side cooling that are suitable for use by RF devices and circuits.